JANUARY 20, 1921
receiver, some of which can pick up the sound of under-
water propellers at a distance of 15 miles. On the other hand,
the noise of an aircraft engine and propeller, even when the
aircraft is on the sea close to a submarine, cannot be heard
on the submarine's hydrophone. The advantages thus
gained are obvious, and Col. Strain pointed out that if a sub-
marine is located in a particular area, a flight of four seaplanes
can be sent out, three spreading themselves out at extreme
hydrophone ranges, say about five miles apart, the fourth
remaining in the air to attack as directed and to work the
wireless. In an hour they can make a hydrophone search of
an area of 400 square miles without the submarine knowing
that she is being hunted, unless she comes to the surface,
in which case the machine in the air has a chance of sighting
and bombing her. If the machines on the sea hear a submarine,
the three should be able to get a bearing of the sound and
plot the position of the submarine with sufficient accuracy to
taxy over the spot and depth-charge her.
The lecturer then mentioned the advent of the " Large
America " type flying boat, which, with her twin-engines,
gave the observer an excellent view, and had a better range
and lifting capacity. Regarding one of these boats, the lec-
turer told of how she came down in the Heligoland Bight during
very bad weather, so bad that the Harwich light forces could
not go out and search for her, and she was given up as lost.
Her crew, however, rigged up a sea anchor to which she rode
for three days, drifting across the North Sea. When the gale
moderated she got her engines going and taxied into harbour
under her own power. The submarines of those davs, the
Jecturer said, had periscopes which could not see more than
30 degrees from the horizontal. Moreover the ' were thick
and left a very noticeable " feather " in the water. Conse-
quently the earlier large flying boats sank six submarines
and damaged others before the enemy realised this dis-
advantage.
As usual, however, the improvement in attack was followed
by improvement in defence. The enemy periscopes were made
thinner, leaving a much smaller " feather," and the submarines
were provided with " altiscopes," by means of which they could
examine the sky before coming to the surface. Gradually the
areas of submarine activity by day became farther removed
from air stations, and in the areas within reach from these
stations work was mostly confined to the night time. This
led to the use of floating bases from which the seaplanes could
operate, and in the night time the use of aircraft was mostly
confined to listening on the hydrophones and dropping para-
chute flares which illuminated a large area and thus com-
pelled the submarines to submerge, which prevented them from
getting into position for attacking shipping and, more impor-
tant still, prevented them from recharging their batteries.
In this manner the average length of cruise of submarines
operating on the East Coast was reduced from seven days
to five days.
On the question of armament of anti-submarine aircraft
Col. Strain pointed out the two m^in conditions arising,
i.e., the submarine on the surface, and submerged In
the former case, he said, bombs which were fused to detonate
below the surface might not injure her, even if they make direct
hits. He mentioned two cases where bombs with time fuses
made direct hits on submarines on the surface, splintering
without doing much damage. If the submarine has sub-
merged the problem is naturally more complicated. " She
may," Col. Strain said, " have submerged to periscope depth
only, about 26 feet, or she may go to her maximum diving
depth of about 180 feet." Then there is the question of how
far ahead to drop the bombs. This will of course depend on
several things, such as the depth of the submarine, whether
she is still on her dive, in which case her forward speed will be
small, whether or not she has taken up her dive, and whether
she is likely to hold her course or turn to right or left. The
only satisfactory method is, in the opinion of the lecturer,
to strew the whole area with bombs fused to detonate at all
depths. For this purpose patrols should be undertaken by a
flight of weight-carrying aircraft working together rather than
by single machines. •»
The lecturer referred to the advantage of close co-operation
between aircraft and surface-hunting craft, which can carry
a greater weight of explosives than can present-day aircraft.
The three essentials for success in the co-operation are given
by Col. Strain as " A knowledge of the powers and deficiencies
of the arm with which one is co-operating ; adequate signalling
between the two arms, and perfect trust in the other arm know-
ing and carrying out its own job." Finally the lecturer
referred to the organisation and to the development of the
intelligence system, which during the latter part of the War
enabled a much better idea to be formed of where the sub-
marines would operate at a given time, how many there would
be, and how long each was likely to remain out before having
to return to its base.
Turning to the use of aircraft for defence against submarines,
the lecturer dealt with the work of escorting surface craft,
locating minefields and destroying mines which had come to
the surface. He then gave the following figures for the year
1918 : "4,869 escort flights were undertaken by seaplanes
and aeroplanes. Only two vessels were attacked while under
escort, and on one of these occasions the observer saw the
track of the torpedo and was able to signal and warn the ship
in time for her to alter her course and avoid the torpedo.
2,141 escort flights were made by airships, only one vessel
being attacked while so escorted, and on that occasion the air-
ship was five miles away from the ship. 131 escorts were made
with kite balloons towed from surface escorting craft, during
which three vessels were attacked. On two of these occasions
the approach of the torpedo was observed from the kite
balloon."
It was pointed out that the success of aerial escort is not so
much due to aircraft being able to prevent attack as to the fact
that the track of the torpedo is very apparent from the air.
The knowledge of the submarine commander that as soon as he
makes an attack the position where the torpedo track begins
will be bombed, acts as an excellent deterrent. The super-
iority in certain respects of airships over seaplanes and aero-
planes for this work was mentioned, notably that of longer
endurance and ability to cruise slower around the convoy.
On the other hand the airship takes longer to get over the
torpedo track and bomb it. On balance, however, it appears
that Col. Strain rather regards the airship as a useful scare-
crow, as he states that only one submarine was sunk and one
damaged by an airship. The lecturer than gave the following
statistics :—
" For example, take the aircraft operating on the north-
east coast of England from July 1 to September 30, 1918.
There were 58 days on which no flying was done by airships.
On these days 40 attacks were made by submarines on British
shipping, and 29 sightings, mostly followed by attack, were
made by seaplanes and aeroplanes on enemy submarines.
Airships in that area only sighted submarines on four occasions
during the three months."
The defensive value of aircraft in preventing attack on
shipping, locating minefields and diverting traffic from danger
zones cannot be assessed in figures. No one can hazard a
guess at the numbers of ships saved or the tonnage of supplies
brought safely to Britain because of the work of aircraft.
It must be very large. Other results are known and give some
indication of the value of aircraft in anti-submarine warfare :
in 1917 submarines were sighted from aircraft on 169 occasions,
of which they were attacked on 106. In the nine months of
1918 submarines were sighted 192 times, of which 130 resulted
in attack.
It must be remembered that in 1918 aircraft were much more
devoted to escort work, which accounts for the greater mileage
flown for every submarine sighted. As an indication of the
expansion of the work, the distance flown by aircraft on
submarine patrol in home water in 1917 was 1,526,475 miles,
while in the nine months of 1918 this had increased to
4,801,347 miles, nearly three times as much. Prior to 1917
only one submarine was known to have been seriously
damaged by aircraft. That was in 1915. In 1917 seven were
sunk and fifteen seriously damaged by aircraft. Six of the
submarines sunk, viz. :—" U.C. 36," on May 20 ; " U.C. 1,"
on July 24 ; " U.B. 20," on July 29 ; " U.B. 32," on August
18 ; " U.C. 72," on September 22 ; and " U.C. 6," on Septem-
ber 28, were sunk by "Large America" seaplanes unaided. One
submarine, " U. 69," was sighted from a kite balloon towed by
Patriot, which was directed where to drop her depth charges
and sank the submarine.
" In 1918, 4 submarines were sunk, 6 seriously and 20 slightly
damaged by aircraft. All the submarines sunk were finished
off in co-operation with surface craft. They were :—
" LT.C. 49," on May 31, and " U.C. 70 " on August 28, both
of which were found by fairly fast aeroplanes with a good view
ahead. " U.B. 83 " was sighted from Ophelia's kite balloon,
which directed her over the spot, and " U.B. 115 "was sighted
from airship " R. 29 " on September 29, and sunk by her in
co-operation with destroyers and trawlers."
" Everyone engaged in the anti-submarine campaign will
always regret that the ultimate test of their organisation was
not carried through. In October, 1918, the tracking depart-
ment of the Intelligence Division traced a hiving homewards
of enemy submarines and their concentration in the North
Sea off the entrance to the Firth of Forth. From this and
other indications it was apparent that the concentration of
submarines was to play an important part in Germany's last
gamble, a general Naval action.
•y- 47